A loudspeaker is a transducer that produces sound in response to an electrical audio signal input. The vast majority of loudspeakers in use today are electromagnetic transducers. Referred to as dynamic loudspeakers, that class has essentially remained unchanged since the 1920's. Typically, a linear motor, such as an electromagnetic or electrostatic motor, actuates a diaphragm, which causes sound waves to be emitted by the speaker.
As a conventional speaker diaphragm is oscillated, it produces sound waves by moving air. The diaphragm produces two, out of phase sound patterns because as it moves forward, pushing air, it is simultaneously pulling air from behind. A speaker that emits sound in that manner is known as a dipole speaker. Dipole speakers create a diffuse, open sound and are typically used as surround speakers in home theater setups for ambient sound effects. Since dipole speakers send out of phase sound in opposite directions to reflect off of various walls and surfaces, they are often considered unsuitable when precise sound localization or image specificity is desired, such as in the accurate reproduction of music. In order to combat the natural dipole action of a speaker diaphragm, conventional speakers must be placed in bulky enclosures to limit and absorb the sound emitted from the back of the diaphragm and create monopole speakers. Those speakers however, lose the wide dispersion characteristics of a dipole speaker, requiring still more bulky speakers and enclosures to distribute sound.